Aluminum (“Al”), the most abundant metallic element in the earth's crust, is a light weight, silver metal. Its atomic weight is 26.9815, and its specific gravity is 2.7. The element melts at 660° C. and boils at 2467° C. In today's explosives and ordnance industries, aluminum is used in its powder form in explosives and propellants due to the high heat value it generates when it reacts with oxygen. The heat released by oxidizing 1 gram of aluminum into aluminum oxide is 30.95 KJ, compared to the detonation heat of some most often used high explosives, for example, the tested detonation heat of RDX (Hexogen, Cyclotrimethylenetrinitramine) is 6.32 KJ/gram, and that of HMX (Octogen, Cyclotetramethylenetetranitramine) is 6.19 KJ/gram. Aluminum-oxygen reactions are widely used in metallurgy, fireworks, metal welding and in various other industries. When aluminum powder is mixed with a main explosive such as RDX, TNT (Trinitrotoluene), HMX or ANFO (Ammonium Nitrate Fuel Oil, an explosive used in rock blasting), it reacts with the detonation products from the main explosives such as H2O and CO2, giving off extra heat to do useful work. The addition of aluminum powder in propellants increases the heat generated by combustion of a propellant and helps to stabilize the combustion process.
The present invention uses aluminum's reactivity in its molten form with some commonly seen oxygen-carrying chemicals like water or metal oxides. When Al is heated to above its melting point (660° C.), it reacts with water and gives off a large amount of energy. In such a reaction molten aluminum is fuel, and water functions as an oxidizer. Such a reaction proves to be a hazard in the aluminum casting industry. Known as steam explosion, it is a leading cause of fatalities and serious injuries among workers and of property damage in the metal-casting industry worldwide. It has been reported that from 1980 through 1995, the aluminum industry experienced several hundred explosions during casting operations. Three devastating explosions occurred in 1986 alone. Technologies have been developed to suppress such reactions from happening in the workplace and will not be discussed here. The present invention is concerned with the exploitation of such a reaction to do useful work in engineering. The intentional use of aluminum-water reaction for engineering purposes is rarely seen in today's industries. However, there are some patents that involve the use of such a reaction. For example, U.S. Pat. Nos. 4,280,409 and 4,372,213 to Rozner et al. disclose a molten metal-liquid explosive device and method. The patents teach the use of a pyrotechnic mixture such as a metal-oxidizer mixture that upon ignition heats a solid metal liner that in turn reacts with water to create an explosion event.
There are some patents concerning the use of the aluminum-water reaction to launch projectiles in the ordnance industry. U.S. Pat. No. 5,052,272 to Lee discloses the use of aluminum powder/water reaction to generate hydrogen gas and use it to propel projectiles. U.S. Pat. Nos. 5,712,442 and 5,789,696 to Lee and Ford describe the use of an aluminum (or aluminum-lithium, aluminum-magnesium) wire placed in water and be energized by electrical energy, reacting with water to generate hydrogen gas and to launch a projectile.
Recently, researchers at Oak Ridge National Laboratory in the United States found that the aluminum-water mixture can be used as a propellant to replace commonly used gunpowder. According to Dr. Taleyarkhan, the ORNL program manager, when aluminum mixes with water at high temperatures, the aluminum combines with the oxygen atoms in the water, releasing hydrogen and a great deal of energy, potentially four times greater than TNT. The aluminum-water mixture has been used as a new propellant for a specially made gun by the ORNL. The speed of the bullet launched by this gun is adjustable by controlling the strength of the reaction that launches the bullet, turning from deadly force into minor injury and saving lives. The new weapon fueled by aluminum-water mixture is said to be very suitable for law enforcement and defense, as disclosed in U.S. Pat. No. 6,142,056 to Taleyarkhan.
U.S. Pat. No. 5,859,383 to Davison et al. discloses a method to construct an explosive device such as a shaped charge for oil well casing perforation. The device uses energetic, electrically activated reactive blends such as an aluminum-water blend in place of high explosives, and the said reactive blends are activated by inputting electric energy through electric leads. According to the inventors of that patent, the electrically activated reactive composites such as an aluminum-water blend are potentially safe, energetic, environmentally benign alternatives to conventional explosives. Practical devices will contain filaments, foils, or sintered particles with dimensions of approximately 10 microns. They will be activated by electrical pulses produced by capacitors or by generators driven rapidly rotating devices.
In the oil and gas industry, an explosive device called a shaped charge or oil well perforator is used to establish a communication channel between the oil well and a hydrocarbon bearing formation. Typically, the device comprises three parts, namely a machined steel case, a generally cone-shaped liner and a certain amount of explosives sandwiched between the case and the liner. The liner turns into a high velocity metal jet upon detonation of the explosives, penetrating through the steel casing of the oil well, the concrete lining and into the formation. The perforation created in such a manner bears a layer of material hardened by the perforating process. Often called a “crushed zone”, this layer hinders the flow of hydrocarbons into the oil well. Its permeability is much lower than that of the formation in its virgin state. To improve the oil flow, the crushed zone needs to be broken down using different stimulation techniques, including acidizing, hydraulic fracturing and fracturing using explosives or propellants.
Well stimulation using explosives has a long history. According to Watson, S. C. et al., as early as 1864, E. L. Roberts applied for a patent for increasing oil well productiveness with gun-powder explosions (U.S. Pat. No. 47,485, 1865, details unavailable). The patent also includes the use of NG (nitro-glycerine) because its velocity of detonation was 5˜10 times faster, and its shattering effect was much greater, allowing the creation of more fissures through which the oil flowed into the well. Another purpose of explosives stimulation is to remove the paraffin that would clog the perforations after the well is put into production for some time. The heat generated by the detonation of explosives (or the combustion of propellants) melts the paraffin, removes it and cleans the perforations, increasing production.
A major problem with explosives stimulation is the shattering effects on the well. Due to the high detonation velocity and high percentage of shock wave energy associated with high explosives, a great area is crushed and sloughs into the well. Therefore, it generally needs lengthy cleanout time after the shot to resume production. According to Stoller, H. M., explosive fracturing creates a highly fractured region around the well bore; the gas pressure extends a few of these fractures further into the reservoir. The extremely high pressure results in permanent rock compaction and a very low permeability barrier at the well bore. Due to the shattering effects of an explosive event, explosive fracturing is suitable for uncased wells only. In practical applications, it has been realized that the highly dynamic process of explosive stimulation has an overly rapid pressure rise time, and too much shock energy is transmitted into the formation, creating a large quantity of small cracks.
The other method commonly used in well stimulation is hydraulic fracturing. Compared to the highly dynamic explosive fracturing, the loading process of hydraulic fracturing is much slower and can be regarded as a quasi-static process. It needs lengthy setup time and the operating cost is high. Nevertheless, it generally creates only a single crack into the formation from a perforation. Based on a comparison of the advantages and disadvantages between explosive stimulation and hydraulic fracturing, it is apparent that a process that can be used to create a network of multiple fractures with an operating cost similar to that of explosive stimulation would be most desirable and such a process would be associated with the use of propellants. It is assumed that such a network of multiple fractures is more likely to intersect with far-field natural fractures than the fractures created by explosive or hydraulic fracturing processes.
The original well stimulation technology that uses propellant gas generators to create and extend multiple fractures has been studied and applied in engineering with substantial success. The technology has many names in practical applications, such as tailored pulsed loading, controlled pulse pressurization, high energy gas fracturing, controlled pulse fracturing and dynamic gas pulse loading. When used in oil well stimulation, the basic requirements for the process and the propellant include:    1) The pressure generated by the combustion of propellant should be so that it exceeds the tensile strength but be lower than the compressive strength of the formation to be fractured. Also the pressure must be lower than the safety pressure of tubular goods, packers and valves;    2) The pressure rise time should allow it to create multiple fractures and to stay in zone but not at a rate in excess of the acceptable loading rate of the well equipment.    3) The generated gas has a volume big enough to extend the fracture to an effective length.
Propellant used in place of high explosives has been found to be the most suitable to create such a network of multiple fractures in the formation. There are numerous patents concerning the use of propellants in stimulating subterranean hydrocarbon bearing formations as well as the efforts to perforate and stimulate a formation in a single operation (to complete perforating and stimulating of a hydrocarbon bearing formation concurrently). Cited below are just some examples.
U.S. Pat. No. 5,775,426 to Snider et al. describes a method to use perforating charges and propellant stimulation simultaneously. Shaped charges are loaded in a perforating gun and a shell, sheath or sleeve of solid propellant material is used to cover the exterior of the gun. Upon detonation of the charges, the high velocity jets penetrate through the gun, the casing and into the formation. At the same time, the jets, high pressure and high temperature ignite the propellant. The high-pressure gas generated by the combustion of the propellant is forced to enter into the perforations created by the jets, creating multiple fractures from each perforation.
U.S. Pat. No. 4,253,523 to Isben discloses the use of shaped charges in a perforating gun which is filled with secondary explosives with lower detonation velocity. According to the inventor, upon detonation of the shaped charge in the gun, it penetrates into the formation, creating a perforation. The shock wave of that secondary explosive will follow the perforation and will continue through the constant diameter perforated cavity.
U.S. Pat. No. 4,391,337 to Ford et al. describes an integrated jet perforation and controlled propellant fracture device and method for enhancing production in oil and gas wells. The device is loaded with perforating charges and fuel packs. Upon detonation of the perforating charges, the fuel packs are ignited. Then the high-velocity penetrating jet is instantaneously followed by a high-pressure gas propellant such that geological fracturing initiated by the action of the penetrating jet is enhanced and propagated by the gas propellant.
U.S. Pat. No. 4,064,935 to Mohaupt provides a gas generating charge that is placed in the oil well bore and activated to generate a controlled surge of gas pressure-volume of a known magnitude-time profile and directed perpendicular to the side of the well bore to flush clogged material away from the well bore and open up clogged passages for the greater flow of the oil into the well bore without damaging the well.
U.S. Pat. No. 5,690,171 to Winch et al. describes a device comprising a pipe having a plurality of weakened portions and containing a propellant material. When the propellant is ignited it produces rapidly expanding gaseous combustion products that puncture the weakened portions of the pipe. The expanding gas fractures the surrounding formation, thereby stimulating the formation to production.
U.S. Pat. No. 5,355,802 to Petitjean describes a method to perforate and fracture a formation in a single operation. The method includes the use of propellant canisters and shaped charges in a perforating tool, and the proper procedures of igniting the propellant and detonating the shaped charges.
U.S. Pat. No. 5,551,344 to Couet et al. discloses the use of propellant or compressed gas along with a liquid column. Upon ignition of the propellant or the activation of the compressed gas, the high-pressure gas released drives the liquid into the formation to propagate the fracture.
U.S. Pat. No. 4,081,031 to Mohaupt describes the use of a chemical gas generating charge activated to provide a controlled surge of gas pressure-volume of a known magnitude-time characteristic and directed to flush away clogged material in the well-bore and open-up clogged passages for the greater flow of oil into well bore without damaging the well.
U.S. Pat. No. 4,683,951 to P. Pathak et al. discloses a method to enhance the effective permeability of subterranean hydrocarbon bearing formations by proceeding the surfactant fluid injection step with creation of multiple formation fractures using tailored pressure pulses generated by propellant canisters disposed in the injection well. Fluid injectivity rates are increased by subsequent fracture extensions provided by repeated steps of generating high-pressure gas pulses at selected intervals.
U.S. Pat. No. 3,747,679 describes the use of a liquid explosive that has a small critical diameter, is safe to handle to fracture well formation for enhancing well productivity.
U.S. Pat. No. 3,797,391 to Cammarata et al. seems to show an example of the use of aluminum as shaped charge liner material in the purpose to project some liner material into the target upon collapse of the liner. Disclosed by Cammarata et al. is a multiple shaped charge bomlet having a plurality of shaped charges. Each charge has a bimetallic liner (the air side being the high density metal such as copper and the explosive side being the pyrophoric metal such as aluminum, magnesium, zirconium). The charges have the capability of penetrating hard structures and propelling incendiary particles through the perforations made in the target by the shaped charge jet. Since the referenced patent is used in an environment without the presence of water, the exothermic reaction of the incendiary particles should be between the said pyrophoric metal such as aluminum with oxygen in air, and obviously not with an oxygen carrying liquid like water.
Due to the relatively high cost associated with the use of a propellant in oil well stimulation, there have also been efforts to find a substitute for it. U.S. Pat. No. 5,083,615 to McLaughlin et al. discloses the use of aluminum alkyls to react with water within a confined space. The gas-generating chemical reaction can build up substantial pressure, and the pressure can be used to fracture rocks around a borehole, and hence stimulate water, oil or gas wells in tight rock formations. According to the inventors, the pressure can also be used to fracture coal seams for enhanced in-situ gasification or methane recovery. The aluminum alkyls are organo-metallic compounds of the general formula AlR3, where R stands for a hydrocarbon radical. These compounds react violently with water to release heat and the hydrocarbon gas. Some aluminum alkyls are available commercially at low cost. However, the tendency of the aluminum alkyls to ignite spontaneously in air would make it very difficult to handle in practical applications, and the pressure increase in the order of 3000 psi (210 bars) seems to be too low to fracture most of the rock formations.
U.S. Pat. No. 4,739,832 to Jennings et al. teaches a method for increasing the permeability of a formation where high impulse fracturing device is used in combination with an inhibited acid. The inhibited acid is directed into a well bore contained in the formation. A two-stage high impulse device is then submerged within the acid. After the high impulse-fracturing device is ignited, activating the retarded acid by the heat generated; then the fractures in the formation are induced and simultaneously forcing said activated acid into the fractures.